The art of manufacturing fan blades is quite varied. The method used depends on whether the finished fan blade is to be hollow or solid, and whether it is to be one piece or of multiple pieces secured together. The method used also depends on the desired air-foil shape with simple shapes being suitable for pressing while more complex shapes require molding or laying-up techniques. This invention pertains to the construction of hollow, one-piece, glass fiber fan blades of a rather complex configuration such that resin transfer or injection molding is the method of choice. No internal spars or fillers are necessary to provide additional bending strength during operation. This hollow, light construction also produces a blade that is safer to operate as compared to blades with molded-in spars.
In the past, such hollow fan blades were made in two pieces, the first comprising the air foil shape while the second comprised the blade holder that attached the air foil to the rotating shaft or hub. In some cases (U.S. Pat. No. 4,345,877 to R. C. Monroe) the air foil and the blade holder were made of different material. In other cases (U.S. Pat. No. 4,720,244 to Kluppel et al.) the pieces were made of similar material that were then bonded or riveted together.
In the latter, the blade is formed by wrapping multiple glass impregnated cloth layers around a flexible bag within a mold, inflating the bag, and then forcing resin into the mold so as to impregnate the cloths. Afterwards, the fan blade is cured and the flexible bag is removed. In an attempt to more evenly distribute the resin (so as to avoid resin build-up or resin-rich areas which are likely to crack) and to avoid wrinkles, "spreaders" or rigid forms were installed inside the flexible bag and expanded therein to further support the resin impregnated cloths. This spreader was used inside both the blade holder or attachment end as well as inside the air foil end during molding. The insertion and later removal of the spreaders before joining the blade and holder is labor intensive and limits the minimum size and shape of the neck portion of the fan blade. The spreader greatly increased the quality of the product but necessitated making a blade in two pieces since they had to be removed from the interior of each of the parts.
The typical method of making hollow, one-piece fan blades involves using crude cardboard shapes inside the pressurized flexible bags. This, of course, results in a fan blade of non-uniform wall thickness. Additionally, the cardboard shapes produce a fan blade that is full of wrinkles and also of cracked resin-rich areas because of the shifting of the glass fiber cloth within the mold as resin is injected through the neck region. In most normal cases, the cardboard is unable to precisely hold and locate the cloth layers before resin injection.
It is thus an object of this invention to overcome these deficiencies and to provide a method of manufacturing a hollow, one-piece fan blade that is less labor intensive, is stronger, more aesthetically pleasing to the eye and results in fewer rejects than the methods previously used to construct such fan blades. Another object of this invention is to provide a method that specifically reduces the occurrence of resin-rich areas while also providing support and precise placement and retention of the glass fiber cloths so as to reduce their wrinkling or shifting when the resin is forced into the mold. These and other objects of this invention will become obvious upon further investigation.